PITTSBURGH — It could be called a mechanical miracle, a robot that walks on water. With inspiration from nature and some help from research at the Massachusetts Institute of Technology, a research team led by Carnegie Mellon engineering assistant professor Metin Sitti has built a tiny robot that can walk on water, much like insects known as water skimmers, water skaters, pond skaters or Jesus bugs.

Although it's only a basic prototype, Sitti and other researchers imagine that his water-skimming robot could be used on any still water. With a chemical sensor, it could monitor water supplies for contamination or other toxins; with a camera it could be a spy or an explorer; with a net or a boom, it could skim contaminants off the top of water.

Sitti, who runs Carnegie Mellon's NanoRobotics Lab, said he has long been fascinated by water striders and what it would take to build one.

"I think it is the final challenge of microrobotics if you can make this thing," Sitti said. "It needs to be so light and so compact. Look how this animal stays on the water in that kind of miniature, very lightweight body."

For their size, a half-inch on average, the insects can move. Water striders skim across the water as fast as a meter per second; the human equivalent of going 400 miles per hour. They're also very mobile.

For now, Sitti's robot is little more than a half-inch boxy-body made from carbon fibers and eight, 2-inch steel-wire legs coated with a water-repelling plastic (technically making it a water spider).

It also doesn't have a brain, any sensors or a battery. Its "muscles" are three flat-plate piezoelectric actuators, special pieces of metal that change shape when electricity is run through them. The actuators are powered by wires and controlled by three circuits connected to a power supply.

But it can stand on water, it doesn't float, and can skim backward and forward, propelling itself with two legs that act like oars. Although simple now, Sitti said he could build a more complex water-skimming robot within six months without too much trouble.

"These insects are really dumb," Sitti said. "Think about the economy of power, if you are so small why should you need a brain like us to plan everything. These insects have such simple controls."

Sitti's prototype is especially impressive considering researchers didn't really know how water skimmers actually walked on water until last year.

How water skimmers move
The bugs support themselves on water because they're not heavy enough to break the surface tension of water, like a needle that floats.

It was long thought the insects used their legs to create waves to push themselves forward, like a wave hitting a boat.

In 1993, Mark Denny, a Stanford University marine biologist, pointed out a problem: If water skimmers moved by creating waves, newly hatched water skimmers would be immobile because they weren't strong enough to create waves. In reality, newly hatched water skimmers move just as well as full grown adults.

Last year, Massachusetts Institute of Technology mathematician John M.W. Bush and two graduate students solved the riddle by placing dyes and particles in water and using a high-speed video camera.

Bush and the graduate students discovered that water striders move by pushing down on the surface of water enough to create valleys but not enough to break the surface. The water then bounces back like a trampoline to push the insect forward.

Beside the physics and mechanics of walking on water, Sitti's prototype also shows how far robotics has come with the help of lighter and stronger materials.

"If you had asked us 10 years ago to build a water bug, I don't think we would have done it," said Mark Cutkosky, an engineering professor at Stanford University who has been building roach-like robots.

Sitti's robot weighs about a gram, or half of a dime. And so far, it's cheap. Sitti estimates his spartan prototype cost about $10 in materials to make.